Exercise capacity is frequently reduced in symptomatic patients with nonobstructive hypertrophic cardiomyopathy and has been attributed to an impaired cardiac output response. To investigate the role of stroke volume as a determinant of exercise capacity, we studied 22 symptomatic patients with nonobstructive hypertrophic cardiomyopathy, using invasive hemodynamic measurements at rest and during graded semi-erect bicycle exercise along with radionuclide ventriculography and echocardiography. There was a wide range of exercise capacities (50 to 175 watts). The stroke volume index at rest ranged from 22 to 67 ml/beat/m2 and was a modest predictor of exercise capacity (r = .56, p = <.01). There was no correlation between the rest stroke volume index and the left ventricular ejection fraction, peak left ventricular filling rate, or the maximum left ventricular wall thickness. Of note, the stroke volume index during the early exercise (25 watts) was a particularly strong predictor of exercise capacity (r = .78, p <.001) because of changes in stroke volume index from rest, which included a fall in stroke volume index greater than 10% in 9 patients. The stroke volume index at peak exercise was little changed from the stroke volume index at rest and accounted for a strong correlation between the peak cardiac index and the exercise capacity. We conclude that a limited stroke volume index at rest and especially during early exercise is a major determinant of reduced exercise capacity in symptomatic, nonobstructive hypertrophic cardiomyopathy patients and appears to be independent of resting left ventricular systolic function and diastolic filling as well as magnitude of left ventricular hypertrophy.